The invention relates to a process for shaping of a precipitation hardenable non-ferro alloy into wire rod suitable as starting material for drawing into electrical conductor wire. By a Al-Mg-Si alloy is meant. The alloy is said to be "precipitation hardenable", when it comprises alloying elements which can supersaturate the crystal lattice when the alloy is quenched from a temperature at which these elements are dissolved in the alloy, and which can afterwards be precipitated out of the crystal lattice by means of an ageing treatment at medium temperature, so causing a hardening by precipitation, as well known by those skilled in the art. In general an Al-Mg-Si alloy for electrical conductor wire, has a composition of 0.3 to 0.9% of magnesium, 0.25 to 0.75% of silicon, 0 to 0.60% of iron, the balance being aluminium and impurities (i.e. elements in a quantity of less than 0.05%).
In order to give the alloy the final wire form, this alloy is in general hot and/or cold worked. Hot working is working at a temperature where the structure can recrystallize according as it is worked, whereas cold working is working below that temperature. For the finally obtained electrical conductor wire it is also desirable to obtain certain optimal properties, i.e. a high tensile strength coupled with an acceptable and a high electrical conductivity, but with the existing mechanical and heat treatments such property combinations are not always compatible, and the treatments to obtain certain combinations are not always simple. The problems in relation herewith will be explained in relation with the manufacturing of electrical conductor wire made of alloy Al-Mg-Si above, for which the specifications are very stringent in relation to minimum tensile strength, ductility and electrical conductivity in combination, and where there is no large choice in the processes to reduce the wire rods suitable as starting material for drawing into electrical conductor wire which will meet theses specifications.
usually, the manufacturing of a wire of such electrical conductor alloy is in a conventional way conducted in a number of steps: firstly the alloy is entered, either after continuous casting on a casting wheel, or in the form of discontinuous cast bars, into a rolling mill whilst at a hot working temperature of about 490.degree. to 520.degree. C., in order to produce at the exit end of the rolling mill wire rods of a diameter of 5 to 20 mm, in most cases between 7 and 12 mm. However, during rolling the alloy has cooled down to about 350.degree. C. This means that the greater part of magnesium and silicon, introduced to conduct a precipitation hardening treatment at the very end of the manufacturing, is already prematurely precipitated and lost for the hardening.
For this reason, the second manufacturing step is a solution treatment after rolling. Bobbins of wire rods are so kept in a furnace for a number of hours at a temperature of 500.degree. to 520.degree. C. for dissolving the precipitates again in the crystal lattice. Immediately thereafter, the bobbins of wire rods, at the solution treatment temperature, are quenched to a temperature below 260.degree. C., in which the structure is stuck in the state where the alloying elements in solution stay in supersaturated solution in the crystal lattice. This quenching temperature is most often room temperature. Subsequently, these wire rods are cold drawn, which gives a high tensile strength, but strongly reduces ductility to an unacceptable level. For that reason, after drawing, the wire is submitted to an ageing treatment with precipitation hardening, by keeping the wire during a few hours at a temperature of about 145.degree. C. This brings ductility to an acceptable level, with a considerable gain of tensile strength, because the loss due to the softening of the dislocated structure is largely compensated by the precipitation hardening. This is the reason why the alloying elements had to stay as much as possible in solution until the end, in order to allow them to participate as much as possible to the precipitation hardening. Additionally, this ageing step, as it removes internal tensions by the rearrangement of dislocations and by expelling the alloying elements out of supersaturation, is very beneficial for improving the electrical conductivity, which dropped during quenching and drawing, due to the increase of internal tensions.
It has been tried to obtain simpler methods whilst obtaining other, but still acceptable property combinations. In particular, this conventional process requires a solution treatment at very high temperature during many hours, and this is an important factor in the cost price, and consequently it has been tried to eliminate this treatment. All these attempts have as a common goal, that at the exit of the rolling-mill the wire would still have such high temperature, that none or only a small part of the alloying elements should already be precipitated, so that the wire rods can directly be quenched at the exit of the rolling-mill and then, most of the alloying elements are still in solution and can participate to the precipitation hardening afterwards. It has so been proposed to use a very high entrance temperature into the rolling-mill, or a very high throughput speed through the rolling-mill, or an intermediate heating between rolling steps. In the first case, the material is too soft for rolling due to some still liquid eutectic compounds between the crystal grains, in the second case the speed is too high for use together with a continuous casting wheel, or other system of feeding the rolling-mill and in the third case the intermediate heating complicates the rolling step.